The number of monarch butterflies wintering in Mexico dropped by 27% this year, reversing last year’s recovery from historically low numbers, according to a study by government and independent experts released Thursday.

The experts say the decline could be due to late winter storms last year that blew down more than 100 acres (40 hectares) of forests where migrating monarch butterflies spend the winter in central Mexico.

Millions of monarchs make the 3,400-mile (5,500km) migration from the US and Canada each year, and they cluster tightly in the pine and fir forests west of Mexico City. They are counted not by individuals, but by the area they cover.

“The reduction in the area of forest they occupied this year is most probably due to the high mortality caused by storms and cold weather last year,” said Omar Vidal, the head of the Mexico office of the World Wildlife Fund. “It is a clear reminder for the three countries that they must step up actions to protect breeding, feeding and migratory habitat.”

Monarch Butterfly (Danaus plexippus) – Photo by Derek Ramsey

Officials estimate the storms in March killed about 6.2m butterflies, almost 7.4% of the estimated 84m that wintered in Mexico, said Alejandro Del Mazo, Mexico’s commissioner for protected areas. The monarchs were preparing to fly back to the US and Canada at the time the storm hit.

While no butterfly lives to make the round trip, a reduction in the number making it out of the wintering grounds often results in a decline among those who return the next year.

The combination of rain, cold and high winds from the storms caused the loss of 133 acres (54 hectares) of pine and fir trees in the mountaintop wintering grounds, more than four times the amount lost to illegal logging. It was the biggest storm-related loss since the winter of 2009-10, when unusually heavy rainstorms and mudslides caused the destruction of 262 acres (106 hectares) of trees.

However, the fight against illegal logging continues. Last week, authorities detained a man trying to truck about a dozen huge tree trunks out of the butterfly reserve, using false papers asserting the trees were diseased and were being removed to reduce risk. In fact, investigators found the trees had been healthy.

The monarchs depend on finding relatively well-preserved forests, where millions of the orange-and-black butterflies hang in clumps from the boughs. The trees, and the clumping, help protect the butterflies from cold rains and steep drops in temperature.

That is why illegal logging in the 33,484-acre (13,551-hectare) nucleus of the reserve is so damaging. Illegal logging in the monarch reserve dropped from almost 49.4 acres (20 hectares) in 2015 to about 29.6 acres (12 hectares) last year.

Another problem has been the loss of milkweed – the plant monarchs rely on for food – in the US and Canada, because of the increased use of herbicides and the conversion of land to development.

The loss of forests in Mexico and milkweed north of the border has proved devastating. This year the butterflies covered only 7.19 acres (2.91 hectares).

Last year, the butterflies covered 10 acres (about 4 hectares), compared with 2.8 acres (1.13 hectares) in 2014 and a record low of 1.66 acres (0.67 hectares) in 2013. That is still well below the 44 acres (18 hectares) they covered 20 years ago.

“We cannot control the climate, but we can do much better in eradicating illegal logging in the reserve and tackling habitat loss in the US and Canada,” Vidal said. “But even if Mexico’s overwintering sites never lose another tree, without food and habitat along the migration routes, the forests will soon bid farewell” to the monarchs.

The butterfly itself is not endangered, but the migration does appear to be. Not all monarchs migrate.